This invention relates to a system for stopping a spindle at a predetermined rotational position, and more particularly to the system which is capable of stopping a spindle at the predetermined rotational position regardless of whether the spindle is rotating in the forward or reverse direction.
Machine tools with an automatic tool changing function are well known in the art. These are machine tools perform machining work automatically also allow tools to be changed, automatically. The tools carried by such machines are changed in the following manner. First, a magazine holding a number of tools is revolved to bring a vacant tool holding portion of the magazine into position directly above a spindle mechanism. The spindle mechanism, which is grasping an old tool to be exchanged for a new one, is then projected forwardly, after which the magazine positioned above the spindle mechanism is lowered to permit the old tool to be received and grasped by the vacant tool holding portion of the magazine. The spindle mechanism is then retracted so that the old tool separates from the spindle, thereby transferring the old tool to the magazine. Next, the magazine is revolved to bring the desired new tool into position in front of the spindle, and the spindle mechanism is moved forward to allow the spindle to receive and to grasp the new tool. Finally, the magazine is raised away from the spindle to complete the tool change operation.
It is necessary in the tool change mechanism of the foregoing type that a prescribed part of the spindle, such as a key member, be stopped accurately at the correct rotational position in order to permit the fitting portions of the spindle and tool to mate with each other smoothly. More specifically, a key is mounted on the spindle and a keyway is formed in the tool in order to mate with the key. The smooth mating of the spindle and tool requires that the spindle be positioned and stopped in order to provide the correct alignment of key and keyway. Meeting the above requirement necessitates a high spindle positioning accuracy of from .+-.0.1 to .+-.0.2 degrees with respect to the angle of rotation of the spindle.
Conventional automatic tool change mechanisms have a photoelectric detector or a limit switch mechanism which detects the rotational position of the spindle key in order to facilitate the smooth mating of the spindle and tool. The arrangement is such that the spindle is brought to a stop at the prescribed position by the application of a mechanical brake which is actuated in response to a signal from the key position detecting means.
The foregoing apparatus not only fails to provide the desired positioning accuracy but also has a stopping mechanism that experiences wear with long use, since the mechanism relies upon mechanical pin or brake means or the like. Such wear, particularly of a brake shoe or pin, makes it progressively more difficult to stop the spindle at the prescribed position, and the result is that the automatic changing of tools cannot proceed smoothly.
Accordingly, there is a need for a control system which is capable of stopping a spindle at a predetermined rotational position with a high degree of accuracy by purely electrical means, without relying upon a mechanical pin or mechanical brake mechanism to stop the spindle while performing an automatic tool change operation.
In addition, numerically controlled machine tools are being applied increasingly to the boring of such workpieces as automobile engine boxes. Such boring work requires the use of thicker boring bars or cutters in order to prevent chatter by increasing the boring bar rigidity. There are cases, however, where the use of thicker diameter boring bars cannot be avoided, even though the machine is boring through a hole whose dimensions are such that the thicker boring bars cannot be further inserted. This point will be elaborated in the following description of a boring machine tool.
FIG. 1 is an illustrative view of a boring machine tool in simplified form. Provided are a headstock 201, a boring bar 202, and a cutting tool 203. A workpiece 204, which is carried on a table 205, includes holes 204a, 204a' for receiving the boring bar 202, and a hollow portion 204b. In the boring machine tool of this type, the cutting tool 203 is inserted into the hollow portion 204b of the workpiece 204 through either the insertion hole 204a or 204a', after which the workpiece is moved relative to the cutting tool to permit the cutting tool to bore the workpiece in a prescribed manner. The holes 204a, 204a', since they are eventually to be covered, are comparatively small in diameter. It is therefore necessary to select a boring bar 202 having a small diameter, as shown in FIG. 2, in order to prevent the cutting tool 203 from contacting the periphery of the hole 204a when the boring bar is inserted into and withdrawn from the interior of the workpiece. The small diameter boring bar experiences chatter during machining thus, a high degree of boring accuracy cannot be achieved.
Proposed methods for solving the above problem are shown in FIGS. 3 and 4, wherein arrangements are adopted that permit utilization with a boring bar 202 of a larger diameter. In accordance with the method of FIG. 3, the center of the boring bar 202 is offset from the center of the hole 204a along the Y-axis when the boring bar is inserted and withdrawn, the cutting tool 203 being positioned so as to coincide with the Y-axis. It should be noted, however, that the choice of the X-axis here is illustrative only. In accordance with the method of FIG. 4, a cutting tool insertion notch 204c is formed in the workpiece 204 as part of with the insertion hole 204a, and the cutting tool 203 is positioned so as to coincide with the notch 204c when the boring bar is inserted and withdrawn. Both methods depicted in FIGS. 3 and 4 reduce chattering or vibration by allowing the use with a boring bar of a larger diameter. When the cutting tool is inserted into the workpiece and when its rotation is stopped after a machining operation, both of the above methods require that the spindle of the machine tool be stopped accurately at a predetermined rotational position which position is aligned with the positive Y-axis in the arrangement of FIG. 3 or with the position of the cutting tool insertion slot 204c in the arrangement of FIG. 4. In other words, in order to eliminate chatter and effect a rigid machining operation by employing a boring bar of a larger diameter, a control system is required by which the spindle holding the boring bar can be stopped at a predetermined rotational position.
To this end, the assignee of the present invention has already proposed a system for stopping a spindle at a predetermined rotational position through purely electrical means. The immediately above-mentioned system is disclosed in copending U.S. Application Ser. No. 190,659 by Kohzai, et al. entitled "Spindle Rotation Control System", and U.S. Application Ser. No. 215,631 by Fujioka entitled "Control System for Stopping Spindle At Predetermined Rotational System". With this proposed system, however, it is required that the spindle be rotating in a certain direction, such as the forward direction, in order to stop the spindle at the predetermined rotational position. It is not possible to stop the spindle at the predetermined position when it is rotating in the other direction, namely the reverse direction. Accordingly, when the spindle is rotating in the reverse direction, it is necessary to bring the spindle to a halt and then rotate it in the forward direction in order to stop it at the predetermined rotational position. This procedure is combersome and may give the operator a sense of frustration. More importantly, in a boring machine where boring is taking place by rotating the spindle in the reverse direction, a problem is encountered. The problem arises because the cutting tool must be rotated in a direction opposite to that of the machining direction, namely in the forward direction, at the conclusion of the boring work in order to properly orient the cutting tool so that it may be withdrawn from the hole in the workpiece. The problem caused is that the cutting edge of the tool may contact the workpiece in a direction opposite to that of the cutting direction and produce a flaw, such as a kerf, in the side wall of the bore. Since the flaw lowers the value of the article it is necessary to remove the flaw by a costly and time-consuming grinding process. Still another drawback is the considerable time required to finally stop the spindle at the predetermined rotational position. This is because the spindle, which is rotating in the reverse direction in the above example, must first be halted, rotated again in the opposite (forward) direction, and then brought to a stop at the predetermined position. Since this troublesome operation must be repeated for changing tools and for inserting and withdrawing the boring bar, considerably machining time is lost so that the overall machining efficiency declines.